Active light pens generate a light signal in the infrared (IR) portion of the electromagnetic spectrum when a nib (i.e., pen tip) arrangement is in contact with a surface.
When a plurality of IR detectors are arranged at spaced apart locations around a surface upon which a light pen is to be used (such as a wall-mounted whiteboard or a table-top collaboration space), the point at which the light signal is generated may be determined geometrically when the light signal is detected at each of the detectors. The pen tip may thus be tracked as it moves across the surface and the resulting track maybe captured and superposed on a projected image presented on the writing surface. Thus movements of the light pen (e.g., handwriting, diagrams, drawings, etc.) are translated into virtual pen marks in a projected image.
Any pointing object, such as a conventional dry-wipe, white board pen or a finger tip may be detected by a different mechanism that also uses infrared light. In one known example an array of semiconductor diode laser sources, emitting coherent beams of light in the infrared, lays down a laser “curtain.”. Typically this curtain is in the near infrared range (i.e., with wavelengths between 0.7 micrometers and 1.3 micrometers (μm), which may be expressed as 0.7×10−6 m to 1.3×10−6 m). Again, spaced-apart IR sensors (whether the same as for detecting light pen movements or sensors dedicated to detect interruptions of the curtain) detect any deflection of the beams and interpret the overall pattern of disruption as a touch on the writing surface.
In many cases the writing surfaces are uneven and it becomes necessary to offset the laser curtain by a distance (normal to the surface) known as the “engage distance.”
As a result of the offset, writing upon the writing surface may become awkward—the curtain sensors may struggle to correctly distinguish between a finger touch and a light pen.
Known solutions require communication between the pens and a projector (i.e., a device responsible for generating and displaying projected images). This communication uses light pulsing or a radio frequency (RF) signal to allow the pen to send information to the projector telling it if the pen down is activated or if the pen is just breaking the laser curtain.